So-called adventitious bursas are encountered frequently, but a review of the literature indicates that they are poorly understood. Most authors disagree as to the pathogenesis and histologic appearance of such bursas.
In 1890 Schuchardt1 stated that hygromas could develop anywhere in connective tissue independently of previously formed connective tissue spaces. He explained the formation of subcutaneous hygromas on the basis of inflammation, since he found fibrin in inflamed connective tissue. He thought that the fibrin was a part of a liquefaction process that continued until the eventual formation of multilocular small sacs and the subsequent degeneration of the separating walls. He said that the peculiar fibrinous inflammatory process could be recognized in old hygromas only in the wall of the sac and could be followed for a considerable depth. In these regions he found that the nuclei were divided and the cells swollen as they are in coagulation necrosis.
BUCK RM, McDONALD JR, GHORMLEY RK. ADVENTITIOUS BURSAS. Arch Surg. 1943;47(4):344-351. doi:10.1001/archsurg.1943.01220160028004